Fuel cells combine oxygen and fuel to chemically generate electricity without combustion. Fuel cells are simple devices that contain no moving parts, consisting essentially of four functional elements: cathodes, electrolytes, anodes and interconnects. Solid oxide fuel cell (SOFC) technology has the distinct advantage over competing fuel cell technologies (e.g. molten carbonate, polymer electrolyte, phosphoric acid and alkali) because of an ability to use fuels other than hydrogen (such as methane, butane or even gasoline and diesel) and a relative insensitivity to CO that can act as a fuel for these cells, but acts as a poison to other fuel cell types. The general design of a SOFC is two porous electrodes separated by a ceramic electrolyte. The oxygen source, typically air, contacts the cathode, for example a lanthanum manganite doped strontium (LSM) or other conventional cathode material, to form oxygen ions upon reduction by electrons at the cathodes metal/metal oxide/oxygen triple phase boundary. The oxygen ions diffuse through the electrolyte material, which is typically a ceramic material that can function as an excellent conductor of oxygen ions at the temperatures at which the cells is used. The oxygen ions encounter the fuel at the anode forming, water, carbon dioxide (with hydrocarbon fuels), heat, and electrons, which are transported from the anode through interconnects to an external circuit and ultimately back to the cathode.
Although SOFCs are, in concept, simple, the identification of efficient materials for the components and design of effective components remain an enormous challenge. The materials not only require the necessary electrical properties, but must be chemically and structurally stable. State of the art SOFCs operate at temperatures of about 1000° C. to achieve sufficiently high current densities and power. The reactivity of the components, with each other and/or the oxygen and/or the fuel, and the inter-diffusion between components presents a challenge at the high temperatures. The thermal expansion coefficients of the materials must be sufficiently matched to minimize thermal stresses that can lead to cracking and mechanical failure. The air side of the cell must operate in an oxidizing atmosphere and the fuel side must operate in a reducing atmosphere.
One of the more common electrolyte materials for fuel cells is yttria-stabilized zirconia (YSZ) which provides stabilized zirconia in the cubic structure at low temperatures and provides oxygen vacancies. Alternative to YSZ for low temperature cells, below 800° C., are doped cerium oxides and doped bismuth oxides. Although these materials have shown promise, neither is particularly robust mechanically in the reducing atmosphere at the anode. Bismuth oxide-based electrolytes have high oxygen ion conductivities that are sufficient for low temperature operations, but require high PO2 levels for sufficient thermodynamic stability. Low PO2 at the anode promotes bismuth oxide decomposition, which can result in failure of the SOFC. Cerium oxide based electrolytes have the advantage of high ionic conductivity in air and can operate effectively at low temperatures (under 700° C.). However, these electrolytes are susceptible to reduction of Ce+4 to Ce+3 on the anode, leading to electronic conductivity and a leakage current between the anode and cathode.
In addition to the need for a superior electrolyte, improvements to the anode and cathode are desirable. A significant improvement to an anode for SOFCs is disclosed in Wachsman et al. PCT Application Publication No. WO/2010/045329, which is incorporated herein by reference. However, although Wachsman et al. teaches the use of an anode functional layer (AFL) that improves the triple phase boundary between the anode, electrolyte, and fuel, the anode has limits to the range of useful operating conditions due to requirements for mechanical stability. Hence, an anode that permits the improved performance disclosed in Wachsman et al., but with even greater mechanical stability is a desirable goal.